Diversity combining circuit



May 20, 1947. M. G. CROSBY 21,420,858

DIVERSITY COMBINING CIRCUIT Filed April 22 1945 v 5 sheets-sheet 1INVENTOR ATTORNEY May 2o, 1947. n

M. @CROSBY DIVERSITY COMBINING CIRCUIT Filed April 22.-, 1943 5Sheets-Sheet 2L INVENTORV BY Wm ATTORNEY May 20, 1947. M, CRQSBYy2,420,868

DIVERSITY COMBINING CIRCUIT Filed April 22, i945 3 sheets-sheet 3INVENTOR Maf/2,4 Y 6 (035 c BY) l l ATTORNEY Patented May 20, 1947DIVERSITY CGMBINING CIRCUIT Murray G. Crosby, Riverhead, N. Y., assignerto Radio Corporation of America, a corporation of Delaware ApplicationApril 22, 1943, Serial No. 483,989

17 Claims. l

rlhis invention concerns diversity reception in which the outputs ofseveral radio receivers are combined to reduce the effects of fading. Inthe system of the presentinvention, the individual receiver outputs arecombined in a common output circuit in such manner that the strongersignal overcomes the weaker signal more rapidly than in prior systems.This feature of the presentinvention reduces the possibility of morethan one receiver in the diversity system contributing to the commonoutput at one time and producing distortion.

Heretoiore, in the art of diversity reception, the detected outputs ofthe receivers have been combined by connecting the diode outputs to acommon resistor. The strongest signal therefore appearing in any onereceiver of the diversity system develops a bias across the commonresistor as a result of which those diodes of the other receivers whichare fed the weaker signals are biased in such manner that they draw areduced current. Consequently, the stronger signal of the variousreceivers of the diversity system takes control of the common output andbiases the weaker signals so that they contribute less energy to thiscommon output. While this selecting action effected by the use of acommon diode resistor in prior art diversity systems is effective, thereis room for improvement because there is a considerable range over whichthe stronger signal received by a receiver in the diversity system doesnot completely switch oii the weaker signal simultaneously received bythe other receivers in the same system. Measurements have indicated thatwith such an arrangement as known in the art, the stronger signal mustbe approximately 1.8 times stronger than the weaker signal before thecontribution from the weaker signal is made to be negligible. Hence,over this range in which both the strong and the weak signals contributeto the common output, there is the possibility of `distortion and otherfading eilects since the modulations received on both the strong andweak signals may arrive out of phase and cancel.

In the system of the present invention, this range over which both thestronger and weaker signals contribute to the common output isconsiderably reduced. tion distortion in the common output is thereforereduced. Measurements have indicated that with the system ci theinvention, the contribution by the weaker signal is negligible when thestronger signal is only 1.16 times the weaker, as compared The.possibility oi cancellato the 1.8 gure required in prior diversitysystems.

A better understanding of the invention may be had in the followingdescription, which is accompanied by a drawing wherein:

Fig. 1 graphically illustrates, for comparison purposes, theinput-output characteristics of the individual receivers of a diversitysystem as arranged both conventionally and in accordance with theinvention; and

Figs. 2 and 3 illustrate, schematically, two

diversity receiving systems embodying the principles of the presentinvention.

Fig. 1 shows, in solid line curves, measured characteristics of theconventional standard diversity system using the common diode resistorand the corresponding selection characteristics, and shows in dash linecurves, measured characteristics of the combining circuit of the presentinvention. The curves of Fig. 1 were obtained by applying carriersmodulated with different modulating tones to the two receivers of atwo-receiver diversity system. The diiierent tones were selected in thecommon output circuit by means of audio lters. Hence, the relativeamplitudes of the tones gave an indication yof the relative amount ofoutput signal contributed by each receiver. For instance, when the inputcarriers had an amplitude ratio of -2 decibels, receiver B of theconventional diversity system contributed a 29.8 decibel signal to theoutput, as indicated, while receiver A of the conventional diversitysystem contributed 24.8 decibels. At this same input carrier ratio, thediode combiner circuit of the present invention gave an output of 37decibels for receiver B and 8 decibels for receiver A. Thus, in the caseof the conventional diversity receiver combination, the stronger signalhad overcome the weaker signal by a difference of 29.8-24.8=5.0decibels. In the case of the diode combiner circuit of the presentinvention, however, the corresponding diference is 37-8: 29 decibels. Itis thus apparent that the selecting action of the diversity system ofthe invention is much more rapid and positive than the conventionaldiversity system.

Fig. 2 diagrammatically shows the diversity system of the invention asit was used in obtaining the curves in dash lines of Fig. 1. The systemof Fig. 2 shows th'ree similar receivers. Since the apparatus at thethree receivers are4 similar and similarly arranged, the units of thesecond and third receivers have been given the same reference numeralsas the iirst receiver, with the addition of prime designations. Theoperation of all three receivers is the same. Each receiver includes anantenna A, A' or A", a radio frequency amplifier B, B or B", a frequencyconverter C, C' or C", and an intermediate frequency amplifier D, D orD. Each receiver may have its own local beating oscillator (not sh'own)connected to the frequency converter; or, if desired, there may be useda common beating oscillator for all three receivers. The output circuitsof the intermediate frequency amplifiers have been designated bytransformers I, I and I", and these transformers' respectively feeddiodes 2, 2' and '22. Transformers I, I and I" pass the intermediatefrequency outputs of the three receivers to be combined. Theseintermediate frequency outputs are rectified by diodes 2, 2' and 2 sothat the respective detected outputs appear on resistors 3, 3? and 3.".Coupled to the output of each intermediate frequency amplifier in eachreceiver there is provided a low pass filter I2, I3 and I4 which removesintermediate frequency energy from the detected output and also acts asa by-pass capacitor across the diode resistor 3.

The rectified outputs from the three receivers are connected throughdiode tubes 4, 4 and 4 to a common point I5 which is the high potentialside of resistor 5. The low potential side of resistor 5 is suppliedwith positive potential by means of potentiometer II. The combinedoutput from the three receivers, which appears across resistor 5, is fedto the control grid of audio amplier 8 through blocking condenser 5 andis made available at jack IU for utilization by anysuitable translationdevice, such as a loud-speaker or head set, or a recorder. Automaticgain control potentials are taken from the high potential side ofresistor 5 and fed over lead AGC to bias the grids of the radiofrequency and intermediate frequency amplifiers. In effect, the diodetubes 4, 4 and 4" function as variable resistors since the diode whichpasses the signal is biased to such a point on its anode currentcharacteristic that it passes current for both positive and negativeapplied input voltages.

In the operation of the diversity system of the invention, let it beassumed for the sake of simplicity that signals are present in only thefirst two of the receivers and appear in transformers I and I. Whenthese two signals are equal in amplitude, equal currents will iiowthrough diodes 4 and 4 so that both receivers will contribute an equalamount to the common output. If the amplitude of the signal intransformer I rises above that of the signal in transformer I', thenegative voltage fed from resistor 3 through diode 4 to resistor 5 willbe greater than that fed from resistor 3 through diode 4 to 5. This willraise the negative voltagel appearing across resistor 5, due to theincreased current flow therein, so that the plate of diode 4 isrelatively more negative than its cathode. Such a condition will cut offthe current through' diode 4'. Hence diode 4 will act asan open circuitand will not pass energy from resistor 3f to the common output resistor5. Thus, the stronger signal from transformer I isleft to furnish theoutput for the audio frequency amplifier 8, while the weaker signal isswitched off.

The operation of the system of the invention curs when a diode isoperated as a detector feedwith signals present in all of the receiversis ing into an audio load which is too low. The adjustment of thispositive voltage on the potentiometer II is not critical to remove thistype of distortion, and thus requires no voltage regulator. Comparisonmeasurements made of distortion in the apparatus of the diversitysysteni of the invention without connection to the antennas show therelative same low value of about 2% distortion which is present in theconventional diversity method of combination. Such measurements weremade by a signal generator to determine the linear characteristics ofthe system, and should not be confused with fading distortion whichoccurs during reception of radio signals.

fn the experimental set up, diode resistors 3, Ii and 3" were 13,000ohms. Diodes 4 were of the RCA GHS type. Output resistor 5 was onemegohm. Potentiometer II was 40,000 oh'ms and was adjusted to apply apositive bias of about 20 volts to the low potential side of resistor 5.

Although the receiving system of Fig. 2 (as shown) is designed forreceiving amplitude modulated waves, the principles of the invention areequally applicable to frequency and phase modulation diversityreceivers, provided that in the last two systems the signals aredetected by suitable apparatus including frequency or phase modulationdiscriminators, without limiters preceding the discriminators. Fig. 3shows only the essential features of such frequency or phase diversityreceivers. The antennas, frequency converters and intermediate frequencyamplifiers of the three receivers have not been shown since they mayfollow the illustration of the system of Fig. 1 and their illustrationis not necessary to an understanding of the system of Fig. 3. If afrequency modulation discriminator is employed, it may be of the typedescribed in my United States Patent 2 295,092, granted September 15,1942, or 2,243,417, granted May 27, 1941. If a phase modulationdiscriminator is employed, it may be of the tyoe described in my UnitedStates Patent 2,233,773, granted March 4, 1941, or 2,204,575, grantedJune 18, 1940. The discriminators of the different receivers arelabeled, respectively, 30, 30' and 3U and respectively feed differentialtype detectors y3i, 32; 3|', 32' and 3i, 32". When diversity receptionis used with frequency modulation or phase modulation detectors, thereshould be no limiter in the receivers. The elimination of the limitersallows the detected outputs to vary in accordance with the signalstrengths so that the combining system may choose the strongest signal.Were limiters used, the detected outputs would all be equal and thecombining circuit would not be able to choose the best signal.

IThe term angular velocity modulated waves" employed in the claims isdeemed to include both frequency and phase modulated waves.

What is claimed is:

1. A diversity receiving system comprising a plurality of spacedantennas, a receiver coupled to each antenna and including an amplifier,a rectifier coupled to the output of said amplifier, a low pass filterin the space current path of the rectifier, a resistor in shunt to saidfilter, a diode tube having its cathode connected to a point on saidresistor for deriving a negative voltage therefrom during the passage ofcurrent through the rectifier, direct connections between the anodes ofthe diode tubes in circuit with the different receivers, a resistorhaving one terminal directly connected in common to the anodes of saiddiode tubes and its other terminal connected to a source of positivepotential, and an audio frequency amplifier having its input circuitcoupled to said resistor.

2. In a diversity receiving system having a plurality of receiversfeeding a common utilization circuit, each of said receivers including aradio frequency amplier, a frequency converter, an intermediatefrequency amplifier and a rectifier for rectifying the output of saidintermediate frequency amplifier, said rectifier in cluding a cathodeconnected directly to ground, a low pass filter shunted by a resistor inthe space current path of said rectifier, said filter being locatedbetween the cathode and another electrode of said rectifier, a diode foreach rectif-ier having a cathode directly connected to that terminal ofsaid resistor which is farthest away from the cathode of said rectifier,means for directly connecting together the anodes of the diodesassociated with the different receivers, a common load resistor havingone terminal directly connected to the anodes of said diodes and anotherterminal connected to a potentiometer, a connection from one end of saidpotentiometer to ground, and a connection from the other end of saidpotentiometer to a positive polarizing potential, and an automatic gaincontrol connection from the diode anode terminal of said common loadresistor to the radio freduency and intermediate frequency amplifiers ofsaid receivers.

3. A diversity receiving system comp-rising a plurality of receivers, anantenna coupled to the input side of each receiver, the several antennasbeing geographically spaced from one another, a common utilizationcircuit for the different receivers, selective means controlled by apredominant value of signal energy fed through any one receiver forinstantly rendering 'that energy predominantly effective in saidutilization circuit, said means being simultaneously and selectivelyeffective to prevent the transfer of signal energy from the remainingreceivers to said f utilization circuit, said means comprising a rectierand an electron discharge tube serially connected between the output ofeach receiver and the common utilization circuit, said electrondischarge tubes each having an output electrode, a direct conductiveconnection between the output electrodes of said electron dischargetubes, whereby the passage of the strongest signal through one of saidrectiers and its associated electron discharge tube provides a biaswhich prevents current from passing through the other electron dischargetubes.

4. A diversity receiving system for angular velocity modulated waves,comprising a plurality of similar receivers each of which includes inthe inator, a common utilization circuit for all said receivers, meansfor` passing to said utilization circuit the signal from only thatreceiver having the largest incoming voltage, said means includingelectron discharge tube units for the differtube units, whereby thepassage of currentt through any one electron discharge tube unitproduces a direct component of voltage drop in said load resistor whichappears as a negative bias on the other electron discharge tube units.

5. A diversity receiving system comprising a plurality of receiverchannels each of which includes a rectifier for rectifying the outputthereof, an electron discharge device resistor for each channel incircuit with the output of the rectier of that channel and adapted topass the rectified current thereof, a common direct current impedanceelement connected to the outputs of said electron discharge deviceresistors of all of said channels, and means for supplying aunidirectional potential to said common impedance element, whereby theflow of current through said impedance element caused by the now ofrectied current passing through one of said rectiiiers and the dischargedevice resistors in circuit with its output produces a bias on the otherdischarge device resistors in such direction as to reduce theirresponsiveness.

6. A diversity receiving system comprising a plurality of spacedantennas, a receiver coupled to each antenna and including an amplifierand a rectifier following said amplifier, said rectifier deriving energyfrom the output of said amplifier, a tube connected to operate as adiode coupled to the output of the rectifier of each receiver, a commonreceiving circuit coupled to the outputs of said tubes, an impedance inthe space current path of each rectifier, and means for biasing eachsaid tube over a path including in series the said impedance of therectifier in circuit with the saine receiver, whereby the flow ofcurrent through a rectifier causes a direct current to flow through theimpedance in its space current path and through the said tube coupled toits output and produces a change in the bias of the tubes in circuitwith the outputs of the other rectiflers.

'7. A diversity receiving system comprising a plurality of receiverchannels each of which includes a rectifier for rectifying the outputthereof, electron discharge device resistors respectively in circuitwith the rectiers of said different receiver channels and adapted topass the rectified currents of the different receivers, each of saidelectron discharge device resistors having an output electrode, a commonresistor having one terminal connected to the output electrodes of saidelectron discharge device resistors, and means for supplying a positivepolarizing potential to the other terminal of said common resistor as aresult of which polarizing potentials are supplied through the commonresistor to all of said electron discharge device resistors, whereby thefiow of current through said common resistor caused by the flow ofrectified current passing through one of said rectifiers and theelectron discharge device resistor in circuit therewith produces a biason the other discharge device resistors in such direction as to reducetheir responsiveness.

8. In a diversity receiving system having a plus rality of receiverchannels feeding a common utilization circuit, means for passing to saidutilization circuit the signal from only that receiver channel havingthe largest incoming voltage, said means including a rectifier in eachreceiver channel for rectifying the alternating current in that channel,and also including an electron discharge device resistor for passing therectied current from the rectier, and a common load resistor for all oftlie discharge device resistors of said channels, whereby the passage ofcurrent through any one of said discharge device resistors produces adirect component of Voltage drop in said load resistor which appears asa negative bias on the other discharge device resistors.

9. In a diversity receiving system having a plurality of receiversfeeding a common utilization circuit, said receivers including amplifierstages, means for passing to said utilization circuit the signal fromonly that receiver having the largest incoming voltage, said meansincluding a rectifier in circuit with each receiver for rectifying thealternating current signal in each receiver and an electron dischargeresisto;1 foi` each receiver for passing the rectified currenttherefrom, a common load resistor for said electron discharge resistorsconnected thereto by a direct current connection, whereby the passage ofcurrent through any one discharge device resistor produces a directcomponent of voltage drop in said load resistor which appears as anegative bias on the other` discharge device resistors, and an automaticgain control connection from a point on said common load resistor to theamplifier stages of said receivers.

10. A receiving system for angular velocity waves, comprising in theorder named a radio frequency amplifier, a frequency converter, anintermediate frequency amplifier, a discriminator and a differentialtype detector for rectifying the output of said discriminator, anelectron discharge device resistor coupled to said differential typedetector, a load resistor connected to the output of said dischargedevice resistor, and a utilization circuit coupled to said loadresistor.

11. A receiving system for angular velocity waves, comprising in theorder named a radio frequency amplifier, a frequency converter, anintermediate frequency amplifier, a discriminator and a differentialtype detector for rectifying the output of said discrimina-tor, anelectron discharge device resistor coupled to said differential typedetector, a load resistor having one terminal connected to the anodeelectrode of said discharge device resistor and its other terminalconnected to a source of positive potential, and an electron dischargedevice utilization circuit having its input coupled to the rst terminalof said load resistor.

12. In a diversity receiving system having a plurality of receiversfeeding a common utilization circuit, each of said receivers including aradio frequency amplifier, a frequency converter, an intermediatefrequency amplifier and a rectii'ler for rectifying the output of saidintermediate frequency amplifier, said rectifier including a cathodeconnected directly to ground, a low pass lter shunted by a resistor inthe space current path of said rectifier, said filter being locatedbetween the cathode and another electrode of said rectifier, a diode foreach rectifier having a cathode directly connected to that terminal ofsaid resistor which is farthest away from the cathode of said rectier,means for directly connecting together the anodes of the diodesassociated with the different receivers, a common load resistor havingone terminal directly connected to the anodes of said diodes and anotherterminal connected to a potentiometer, a connec- 8 tion from one end ofsaid potentiometer to ground, and a connection from the other end ofsaid potentiometer to a source of positive polarizing potential.

13. In a diversity receiving system having a plurality of receiversfeeding a common utilization circuit, a rectifier in circuit with theoutput of each receiver for rectifying the alternating current signal ineach receiver, an electron discharge device resistor in the output ofeach rectier for passing the rectified signal, and a common loadresistor for said discharge device resistors, whereby the passage ofcurrent through any one said discharge device resistor produces acomponent of voltage drop in said load resistor which appears as anegative bias on said other discharge device resistors.

14. In a diversity receiving system having a plurality of receiversfeeding a common utilization circuit, a rectifier in circuit with eachreceiver for rectifying the alternating current signal in each receiver,a variable resistor in the form of an electron discharge tube coupled tothe output of each rectifier for` passing the rectified signal` and acommon load resistor for said electron discharge tubes, whereby thepassage of current through any one of said electron discharge tubesproduces a component of voltage drop in said load resistor which appearsas a negative bias on lthe other of said electron discharge tubes.

15. A diversity receiving system comprising a plurality of receivers, anantenna coupled to the input side of each receiver, the several antennasbeing geographically spaced from one another, a common utilizationcircuit for the different receivers, selective means controlled by apredominant value of signal energy fed through any one receiver forinstantly rendering that energy predominantly effective in saidutilization circuit, said means being simultaneously and selectivelyeffective to prevent the transfer of signal energy from the remainingreceivers to said utilization circuit, said means comprising a rectifierand an electron discharge tube serially connected between the output ofeach receiver and the common utilization circuit, each of said electrondischarge tubes being connected to operate as a diode, said electrondischarge tubes each having an output electrode, a direct conductiveconnection between the output electrodes o-f said electron dischargetubes, whereby the passage of the strongest signal through one of saidrectifiers and its associated electron discharge tube provides a biaswhich prevents current from passing through the other electron dischargetubes.

16. A diversity receiving system for angular velocity modulated waves,comprising a plurality of similar receivers each of which includes inthe order named a radio frequency amplifier, a frequency converter, anintermediate frequency amplifier, a discriminator and a differentialtype detector for rectifying the output of said discriminator, a commonutilization circuit for all said receivers, means for passing to saidutilization circuit the signal from only that receiver having thelargest incoming voltage, said means including individual electrondischarge device resistors coupled to the different differential typedetectors for passing the rectified currents from said detectors, and acommon load resistor for said electron discharge device resistors,whereby the passage of current through any one electron discharge deviceresistor produces a direct component of voltage drop in said loadresistor which appears as a, negative bias on the other electrondischarge device resistors. REFERENCES CITED 1'7- A feceYng ys'em forangular velocit?? The following references are of record in the waves,comprismg 1n 'chie order named radio me of this patent: frequency amplier, a requency couver er, an 5 intermediate frequency ampler, adiscriminator UNITED STATES PATENTS and a differential type detector forrectifyng the Number Name Date output of said discriminator, a diodehaving a 2,004,128 Peterson June 11, 1935 cathode coupled to saiddetector and an anode 2,290,992 Peterson July 28, 1942 coupled to a loadresistor, and a utilization cir- 10 2,268,643 Crosby Jan. 6, 1942 cuitcoupled to said load resistor. 2,286,410 Harris June 16, 1942 MURRAY G.CROSBY.

